Sounds interesting, actually. I'd like to help but I've got tons of personal projects that need completion.
Your problem is two fold:
1. Scraping reddit or otherwise getting the information from a subreddit.
2. Sending messages.
Scraping reddit: you should consider looking at the reddit API: https://www.reddit.com/dev/api/ The API terms are pretty loose.
Screen scraping is a PITA. You should always avoid it whenever possible. Any small UI change is likely to break your entire program. Some UI dipshit decides he doesn't like the placement of a button and BAM, your entire system is FUBAR.
I'd suggest using the API to gather usernames at least.
Messages: You CAN send messages via the API, but it's a pretty good way to get your associated account banned. If you're spamming, you're probably going to lose accounts fast. Here I'd suggest a third-party reddit api + farming out a way of getting new accounts (or if you have the patience, making the accounts yourself).